External information and Generalized Additive Models (GAMs) are used to improve the indices provided by the BTS survey for the stock assessment of plaice and sole. These ancillary data consist of the following variables: Depth, Sediment grain-size, Surface temperature, Latitude, Longitude, Time of day and Day of year. Three approaches that predict the catches of four age-groups of plaice (1-4+) and sole (1-4+) were studied: (1) a “basic” GAM that incorporated the external variables; (2) A GAM where the catches of fish species other than the two target species were represented by three Principal Components (PC's) and added to the “basic” model; (3) The predictions of the basic model were applied to a regular grid covering a slightly expanded index area. The results are validated using two criteria: one is that of internal consistency, the other compares the estimates with the results of the stock assessments of plaice and sole without the tuning of the BTS index. Both in terms of internal consistency and correlation with the stock assessments all three methods involving GAMs performed better than the actual observed catches. The approach where the predictions of the basic model were applied to a grid performed best of all for both plaice and sole.